1. Field of the Invention
The present invention relates to a driving circuit of a liquid crystal display (LCD) device, and more particularly, to a driving circuit capable of reducing a difference between respective input voltages being input into driver integrated circuit (IC) chips.
2. Description of the Prior Art
A thin film transistor display, such as a thin film transistor liquid crystal display (TFT-LCD), utilizes many thin film transistors, in conjunction with other elements such as capacitors and bonding pads, arranged in a matrix as switches for driving liquid crystal molecules to produce brilliant images. The advantages of the TFT-LCD over a conventional CRT monitor include better portability, lower power consumption, and lower radiation. Therefore, the TFT-LCD is widely used in various portable products, such as notebooks, personal data assistants (PDA), electronic toys, etc. Gradually, the TFT-LCD is even replacing the CRT monitor in desktop computers.
Generally speaking, a TFT-LCD includes an upper substrate having a plurality of color filters, a lower substrate, and a plurality of liquid crystal molecules filled between the upper substrate and the lower substrate. Additionally, a plurality of scanning lines and a plurality of signal lines perpendicular to the scanning lines are formed on the lower substrate. At least one thin film transistor, used as a switch device of a pixel, is formed at an intersection of each of the scanning lines and each of the signal lines.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a liquid crystal display panel. As shown in FIG. 1, a liquid crystal display panel 10 comprises a substrate 12 and an X-board 14. The X-board 14 is used for outputting signals into the substrate 12, for making the liquid crystal display panel 10 display an image. Moreover, the liquid crystal display panel 10 further comprises a plurality of tape carrier packages (TCP) 16 that are used for electrically connecting the X-board 14 and the substrate 12. Each of tape carrier packages 16 packages a driver integrated circuit (IC) chip (not shown) thereon.
The substrate 12 comprises a plurality of scanning lines S1-Sm, and a plurality of signal lines D1-Dn perpendicular to the scanning lines S1-Sm. A plurality of pixels (not shown) are therefore defined in an active region 18 by the scanning lines S1-Sm and the signal lines D1-Dn. Additionally, the substrate 12 further comprises an outer lead bonding (OLB) region 20, and a driving circuit 22 positioned in the outer lead bonding region 20. The driving circuit 22 includes driver IC chips 22a, 22b, and 22c that are used for outputting switching or addressing signals into the scanning lines S1-Sm. The above-mentioned driver IC chips packaged in the tape carrier packages 16 are used for outputting image signals into the signal lines D1-Dn.
Moreover, the driver IC chips 22a, 22b, 22c are directly formed on the substrate 12 by use of the chip-on-glass (COG) technology. The driving circuit 22 further comprises a plurality of conductive wires 24 for electrically connecting the driver IC chips 22a, 22b, and 22c. For reducing a production cost, the conductive wires 24 are directly formed on the substrate 12, which is so-called wiring on array (WOA) technology. Thereafter, please refer to FIG. 2. FIG. 2 is an equivalent circuit of the driving circuit shown in FIG. 1. As shown in FIG. 2, an equivalent circuit 30 comprises the driver IC chips 22a, 22b, 22c, and resistors 32a, 32b. The resistor 32a connects the driver IC chips 22a and 22b, and corresponds to the conductive wires 24 located between the driver IC chips 22a and 22b as shown in FIG. 1. Similarly, the resistor 32b is connected between the driver IC chips 22b and 22c, and corresponds to the conductive wires 24 located between the driver IC chips 22b and 22c as shown in FIG. 1.
Referring to FIG. 1, when the liquid crystal display panel 10 displays an image, a voltage pulse of a controlling signal 28 is output from the X-board 14, and then, the voltage pulse is sequentially inputted into the driver IC chips 22a, 22b, 22c through the tape carrier packages 16 and the conductive wires 24. Finally, switching or addressing signals are outputted from the driver IC chips 22a, 22b, 22c, and are inputted to the scanning lines S1-Sm. However, due to the extremely large electrical resistance of the conductive wires 24, a voltage drop on each of the conductive wires 24 is significant. Therefore, when the voltage pulse of the controlling signal 28 is sequentially transmitted to the driver IC chips 22a, 22b and 22c, the respective input voltages being input into the driver IC chips 22a, 22b and 22c are quite different. That is, the respective input voltages being input into the driver IC chips 22a, 22b and 22c vary with the positions of the driver IC chips 22a, 22b and 22c. In order to reduce the electrical resistance of the conductive wires 24, manufacturers currently usually increase a width or a thickness of the conductive wire 24.
Additionally, an insulation layer is formed between each of the scanning lines S1-Sm and each of the signal lines D1-Dn, and further, the scanning lines S1-Sm and the signal lines D1-Dn are made of conductive materials. Accordingly, a parasitic capacitor is formed at an overlapping region 26 of each of the scanning lines S1-Sm and each of the signal lines D1-Dn. As the voltage pulse input into each of the signal lines D1-Dn is changed, the voltage variations on the signal lines D1-Dn will be coupled to the scanning lines S1-Sm through the parasitic capacitors at the overlapping regions 26, thus producing a glitch to disturb the controlling signal 28. That is, due to the parasitic capacitors at the overlapping regions 26, the voltage variations on the signal lines D1-Dn will make the controlling signal 28 distort. Therein, the responding current (I) of the controlling signal 28 versus time (t) comprises both of direct current (DC) and alternative current (AC), as shown in FIG. 3. Unfortunately, the AC part of the responding current causes the respective input voltages being input into the driver IC chips 22a, 22b and 22c to be different. This causes the liquid crystal display panel 10 to display an image having band mura, and reduces a displaying quality of the liquid crystal display panel 10.